1. Field of the Invention
The present invention relates to a driver circuit for light emitting elements connected in series and an optical amplifying repeater using this circuit. More particularly, it relates to a driver circuit used in a light emitting source, which outputs composite optical power of lights emitted from plural light emitting elements.
2. Related Prior Art
FIG. 6 is a diagram for explaining a conventional circuit. In the diagram, to be concrete, a series circuit of multiple light emitting elements, which are, laser diodes LD.sub.1, LD.sub.2, . . . LD.sub.n and a constant current power source 3 in series is connected to a power source V.sub.cc.
PD.sub.1, PD.sub.2, . . . PD.sub.n are light receipt elements, each of which detects optical power emitted from the corresponding light emitting element and, to be concrete, they are photo diodes.
These photo diodes are connected in series to resisters R.sub.1, R.sub.2, . . . R.sub.n, respectively and further connected to the power source V.sub.cc.
Reference numerals "11, 12 . . . 1n" are optical power control circuits (ARC), each having a first input terminal, which is supplied with the optical power level detected by the light receipt element corresponding respectively and a second input terminal which is supplied with a reference potential REF.
T.sub.1, T.sub.2, . . . T.sub.n are bypass current control elements connected in parallel to light emitting elements LD.sub.1,LD.sub.2, . . . LD.sub.n, respectively, to be concrete, which are transistors.
The outputs from the optical power control circuits 11, 12, . . . 1n as the above-described, are inputted to the bases of the bypass current control elements T.sub.1, T.sub.2, . . . T.sub.n, respectively.
Each of the optical power control circuits 11, 12, . . . 1n outputs the control signal having a voltage value, which corresponds to that obtained by subtracting the light receipt level detected by the light receipt elements PD.sub.1, PD.sub.2, . . . PD.sub.n corresponding from the reference voltage REF.
That is, when the light receipt level becomes larger, each of the optical power control circuits 11, 12 . . . 1n reduces the control signal input supplied to the base of the bypass current control elements T.sub.1, T.sub.2, . . . T.sub.n. A negative feed back is given to the optical power of the light emitting elements.
When the respective optical powers from the light emitting elements LD.sub.1, LD.sub.2 . . . LD.sub.n become larger, the respective outputs of the optical power control circuits 11, 12 . . . 1n control to increase the electric currents I.sub.1, I.sub.2, . . . I.sub.n flowing to the bypass electric current elements T.sub.1, T.sub.2, . . . T.sub.n, respectively and to decrease the electric currents ILD.sub.1, ILD.sub.2 . . . ILD.sub.n flowing to the light emitting elements LD.sub.1. LD.sub.2 . . . LD.sub.n, respectively and the optical power therefrom.
Contrarily, when the respective optical power from the light emitting elements LD.sub.1. LD.sub.2 . . . LD.sub.n become smaller, the respective outputs of the optical power control circuits 11, 12, . . . 1n control to decrease the currents flowing through the bypass electric current elements T.sub.1, T.sub.2, . . . T.sub.n, respectively, and to increase the currents flowing through the light emitting elements LD.sub.1, LD.sub.2 . . . LD.sub.n respectively and the optical powers therefrom.
By such the structure, the optical powers from the light emitting elements LD.sub.1. LD.sub.2 . . . LD.sub.n are controlled so as to be constant.
In the above-described structure, for example, in the case where the light emitting element LD.sub.2 becomes inferior, some problems cause as follows.
FIG. 7 illustrates a diagram for explaining these problems.
In FIG. 7, let us now consider the case where the optical power of the light emitting element LD.sub.2 becomes to zero because of disconnection fault.
As the above-described, when the optical power from the light emitting element becomes smaller, each of the optical power control circuits 11, 12 . . . 1n performs the control so as to make the base input of the corresponding bypass current control elements T.sub.1, T.sub.2, . . . T.sub.n smaller and make the electric current flowing through the light emitting elements LD.sub.1. LD.sub.2 . . . LD.sub.n larger.
Accordingly, the output from the corresponding optical power control circuit 12 controls so as to make the electric current flowing through the bypass current control element T.sub.2 to zero and to make the electric current flowing only through the light emitting element LD.sub.2, when the optical power of the light emitting element LD.sub.2 becomes zero because of disconnection fault.
However, as the above-described, no electric current through the light emitting element LD.sub.2 flows because of the light emitting element LD.sub.2 being disconnected. At the same time, the electric current does not flow on the series circuit of other light emitting elements LD.sub.1. LD.sub.2 . . . LD.sub.n and the constant current power source 3, as the electric current flowing through the bypass current control element T.sub.2 is also controlled so as to be zero by the output of the optical power control circuit 12.
As the above-described, in the conventional circuit, when either of light emitting elements LD.sub.1. LD.sub.2 . . . LD.sub.n meets with disconnection fault, the current of the corresponding bypass current control elements T.sub.1, T.sub.2, . . . T.sub.n are also controlled so as to be zero with the control of the optical power control circuit 11, 12, . . . 1n. When either one of the light emitting elements LD.sub.1. LD.sub.2 . . . LD.sub.n can not output any optical power with disconnection as this result, such problem as no optical power from other normal light emitting elements is also able to occur.